Propeller shaft and production method for same

ABSTRACT

A low surface-roughness part is formed at a first tapered part, as a result, the roughness of an opening-edge of a groove part of an internal spline part that opens at the first tapered part is reduced, and surface pressure applied by the opening edge to a tooth of an external spline part can be reduced. As a result, the opening edge of the groove part of the internal spline part can be kept from digging into the tooth, and variation, between products, in the insertion load of a second shaft part can be suppressed.

TECHNICAL FIELD

The present invention relates to a propeller shaft and a productionmethod for the propeller shaft.

BACKGROUND ART

A patent document 1 discloses a known conventional propeller shaft asfollows.

This propeller shaft includes a first shaft connected to a transmission,and a second shaft connected to a differential gear, wherein the firstand second shafts are connected to each other via a so-called collapsemechanism that is implemented by spline fitting. The first shaft iscomposed of a first shaft piece and a second shaft piece, wherein thefirst and second shaft pieces are prepared separately and connected toeach other via a collapse mechanism that is formed by an internal splinepart of the first shaft piece and an external spline part of the secondshaft piece.

PRIOR ART DOCUMENT(S) Patent Document(s)

Patent Document 1: JP 2016-61366 A

SUMMARY OF THE INVENTION Problem(s) to be Solved by the Invention

However, this conventional propeller shaft is given no regard to surfaceroughness of an end face of the internal spline part of the first shaftpiece, the end face facing the second shaft piece. This surfaceroughness causes a problem of dispersing a load of insertion of thesecond shaft piece.

In view of the technical problem about the conventional propeller shaft,the present invention has been made for providing a propeller shaft anda production method for the propeller shaft, which serve to reduce adispersion in insertion load of a component including an external splinepart (i.e. a second shaft piece).

Means for Solving the Problem(s)

According to one aspect of the present invention, a surface of a firsttaper part is formed lower in surface roughness than one of a firstlarger-radius part, a second larger-radius part, and a second taperpart.

This propeller shaft is preferably produced by: a first taper partcutting operation in which a surface of the first taper part is cut bymachining to be lower in surface roughness than one of the second taperpart, the first larger-radius part, and the second larger-radius part; asecond taper part cutting operation in which a surface of the secondtaper part is cut by machining; a first larger-radius part cuttingoperation in which a surface of the first larger-radius part is cut bymachining; a second larger-radius part cutting operation in which asurface of the second larger-radius part is cut by machining; and aninsertion operation in which the second shaft piece is inserted into thetubular section of the first shaft piece via the first end side of thefirst shaft piece.

Effect(s) of the Invention

The present invention serves to reduce the dispersion in insertion loadof the external spline part (the second shaft piece).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal half-sectional view of a propeller shaftaccording to the present invention.

FIG. 2 is a longitudinal half-sectional view of a first shaft pieceshown in FIG. 1.

FIG. 3 is a longitudinal half-sectional view of a second shaft pieceshown in FIG. 1.

FIG. 4 is a schematic view illustrating how internal and external splineparts of a second collapse connection shown in FIG. 1 are fitted witheach other.

FIG. 5 is an enlarged longitudinal half-sectional view of the secondcollapse connection shown in FIG. 1.

FIGS. 6A, 6B, 6C, and 6D are longitudinal sectional views of the firstshaft piece for illustrating operations for processing the first shaftpiece, wherein FIG. 6A shows a first larger-radius part cuttingoperation, FIG. 6B shows a first taper part cutting operation, FIG. 6Cshows a second larger-radius part cutting operation, and FIG. 6D shows asecond taper part cutting operation.

FIGS. 7A, 7B, and 7C are schematic views illustrating an operation ofinserting the second shaft piece, wherein FIG. 7A shows a stage beforethe second shaft piece is inserted, FIG. 7B shows a stage where thesecond shaft piece is being inserted, and FIG. 7C shows a stage whereinsertion of the second shaft piece is completed to put the second shaftpiece in a state of spline fitting corresponding to FIG. 4.

FIG. 8A is an enlarged view of a first taper part of a conventionalinternal spline part, and FIG. 8B is an enlarged view of a first taperpart of the internal spline part shown in FIG. 2.

MODE(S) FOR CARRYING OUT THE INVENTION

The following describes a propeller shaft and a production method forthe propeller shaft according to an embodiment of the present inventionwith reference to the drawings. The propeller shaft is exemplified asone applied to an automotive vehicle conventionally.

In the following description, for convenience, left and right sides ineach drawing are respectively referred to as “front” and “rear” sides,and directions along and around a rotation axis Z of propeller shaft PSshown in FIG. 1 are respectively referred to as “axial direction” and“circumferential direction”.

(Configuration of the Propeller Shaft)

FIG. 1 is a side view of propeller shaft PS, showing whole configurationof propeller shaft PS.

As shown in FIG. 1, propeller shaft PS includes a first shaft 1structured to receive a driving force transmitted from a drive sourcenot shown, and a second shaft 2 structured to rotate integrally withfirst shaft 1 due to the driving force transmitted via first shaft 1.First shaft 1 and second shaft 2 are integrally rotatably connected toeach other via a constant-velocity joint 30.

First shaft 1 includes: a front end integrally rotatably connected to aninput shaft IS via a first cross joint 31 serving as a first shaftjoint, wherein input shaft IS is connected to a transmission not shown;and a rear end rotatably supported by a center bearing CB, whereincenter bearing CB is suspended from a vehicle body not shown by abracket BK well-known.

Furthermore, first shaft 1 is composed of two tubular pieces arranged inthe axial direction: a first shaft piece 11 as a front one and a secondshaft piece 12 as a rear one. First shaft piece 11 and second shaftpiece 12 are connected to each other via spline fitting between aninternal spline part 113 and an external spline part 123 describedbelow. This spline fitting forms a second collapse connection C2described below.

First shaft piece 11 includes: a first tube 111 that has a cylindricalshape comparatively thin-walled and includes a front end welded to firstcross joint 31 by friction welding; and a second tube 112 that has acylindrical shape comparatively thick-walled and includes a front endwelded to a rear end of first tube 111 by friction welding. Second tube112 corresponds to “tubular section” defined in claim 1 of the presentapplication.

Second shaft piece 12 includes: a tubular member 121 that has acomparatively large radius and includes a front end connected to firstshaft piece 11 via the spline fitting; and a shaft-forming member 122that has a comparatively small radius decreasing stepwise toward itsrear end and includes a front end welded to a rear end of tubular member121 by friction welding.

Second shaft 2 includes a rear end integrally rotatably connected to anoutput shaft not shown via a second cross joint 32 serving as a secondshaft joint, wherein the output shaft is connected to a differentialgear not shown.

Constant-velocity joint 30 includes: an outer wheel 31 that has atubular shape and is provided at a front end of second shaft 2; an innerwheel 32 that has a tubular shape, and is provided around an outerperiphery of a rear end of shaft-forming member 122, to face an innerperiphery of outer wheel 31; and a plurality of balls 33 rollablyinterposed between outer wheel 31 and inner wheel 32.

Outer wheel 31 includes axial grooves 310 in its inner periphery,wherein each axial groove 310 is formed by cutting to have a straightrecessed shape extending in the axial direction. Axial grooves 310 allowa relative movement between outer wheel 31 and inner wheel 32 in theaxial direction along with rolling of balls 33 in axial grooves 310, andrestrict a relative movement between outer wheel 31 and inner wheel 32in the circumferential direction by engagement between balls 33 andaxial grooves 310.

Radially inside the inner wheel 32, a shaft insertion hole 321 is formedto extend through along an axis of inner wheel 32. Inner wheel 32includes an internal spline part 322 in its inner periphery. Internalspline part 322 is formed by cutting to extend in the axial directionand be fitted with an external spline part 124 inserted in the axialdirection, wherein external spline part 124 is formed by cutting in anouter periphery of the rear end of shaft-forming member 122. Inner wheel32 further includes axial grooves 320 in its outer periphery, whereineach axial groove 320 is formed by cutting to extend in the axialdirection, similar to axial grooves 310 of outer wheel 31.

External spline part 124 of shaft-forming member 122 and internal splinepart 322 of inner wheel 32 form a first collapse connection C1 servingas a so-called collapse mechanism. When the vehicle collides withsomething, this mechanism allows shaft-forming member 122 and innerwheel 32 to relatively move in a shortening direction via both splineparts 124 and 322 under a collision load inputted in a longitudinaldirection, and thereby suppresses propeller shaft PS from being bent.

FIG. 2 is an enlarged longitudinal half-sectional view of second tube112, showing internal spline part 113 and its vicinity of second tube112.

As shown in FIG. 2, internal spline part 113 is formed by cutting toextend in an inner periphery of second tube 112 in the axial direction,wherein second tube 112 is a tubular section of first shaft piece 11provided at a first end side (a rear end side) of first shaft piece 11.Internal spline part 113 has a tooth trace parallel to rotation axis Z(see FIG. 4), wherein both ends of internal spline part 113 in the axialdirection are tapered.

Specifically, second tube 112 includes a first taper part 114 at a firstend (or rear end) of internal spline part 113. First taper part 114 isformed such that: an inner radius Rx of second tube 112 graduallyincreases in first taper part 114 from the rear end of internal splinepart 113 toward the rear end side of second tube 112, wherein innerradius Rx is a distance from rotation axis Z to an inner periphery ofsecond tube 112; and grooves 113 a of internal spline part 113 areopen-ended at first taper part 114.

First taper part 114 is further formed to have a minor angle θ1 lessthan a minor angle θ2, wherein: minor angle θ1 is a minor one ofrelative angles formed by first taper part 114 and rotation axis Z; andminor angle θ2 is a minor one of relative angles formed by a secondtaper part 116 described below and rotation axis Z.

Second tube 112 further includes, in its inner periphery, a firstlarger-radius part 115 closer to the first end side (rear end side) ofsecond tube 112 in the axial direction than first taper part 114. Firstlarger-radius part 115 is formed to have an inner radius R1 greater thana distance R3 from rotation axis Z to grooves 113 a of internal splinepart 113.

Moreover, second tube 112 includes second taper part 116 at a second end(or front end) of internal spline part 113. Second taper part 116 isformed such that: inner radius Rx gradually increases in second taperpart 116 from the front end of internal spline part 113 toward a frontend side of second tube 112; and grooves 113 a of internal spline part113 are open-ended at second taper part 116.

Furthermore, second tube 112 includes, in its inner periphery, a secondlarger-radius part 117 closer to a second end side (the front end side)of second tube 112 in the axial direction than second taper part 116.Second larger-radius part 117 is formed to have an inner radius R2greater than distance R3 from rotation axis Z to grooves 113 a ofinternal spline part 113.

Both of first taper part 114 and first larger-radius part 115 aremachined through a sequence of cutting operations described below usinga common cutter TL. Similarly, second taper part 116 and secondlarger-radius part 117 are machined through a sequence of cuttingoperations described below using the same cutter TL.

First taper part 114 includes, in its surface, a lower-surface-roughnessportion 114 a formed by cutting to be lower in surface roughness thanone of first larger-radius part 115, second larger-radius part 117, andsecond taper part 116. The surface roughness of lower-surface-roughnessportion 114 a is set to a so-called arithmetic mean roughness value Raof 6 or less (Ra≤6), lower than that of a third taper part 125 describedbelow. Lower-surface-roughness portion 114 a is formed by machining at afeed speed slower than that of a cutting operation for the second taperpart described below, or a cutting operation for the first larger-radiuspart described below, or a cutting operation for the secondlarger-radius part described below.

FIG. 3 is an enlarged longitudinal half-sectional view of tubular member121, showing external spline part 123 and its vicinity of tubular member121.

As shown in FIG. 3, external spline part 123 is formed by cutting in anouter periphery of a first end side (or front end side) of tubularmember 121, wherein the first end side of tubular member 121 isstructured to be inserted into second tube 112 via the rear end side offirst shaft piece 11, and wherein external spline part 123 is structuredto be fitted with internal spline part 113 in the axial direction.

External spline part 123 includes third taper part 125 at its first end(or front end). Third taper part 125 is formed such that: an outerradius Dx of tubular member 121 gradually decreases in third taper part125 from the front end of external spline part 123 toward the front endside of tubular member 121; and grooves 123 a of external spline part123 are open-ended at third taper part 125.

FIG. 4 is a schematic view illustrating how internal spline part 113 andexternal spline part 123 of second collapse connection C2 are fittedwith each other.

As shown in FIG. 4, external spline part 123 is formed to have a toothtrace inclined with respect to rotation axis Z, in contrast to internalspline part 113 described above. In other words, internal spline part113 and external spline part 123 are configured to have a relative angleθx formed between a direction of the tooth trace of internal spline part113 with respect to the rotation axis Z and a direction of the toothtrace of external spline part 123 with respect to the rotation axis Z.

Due to this configuration, internal spline part 113 and external splinepart 123 are fitted with each other along with a torsional deformationof teeth 123 b of external spline part 123 in a direction of its tooththickness, wherein the torsional deformation corresponds to theinclination of external spline part 123. In this situation, each tooth123 b of external spline part 123 is inserted (press-fitted) in groove113 a of internal spline part 113 within a region of elasticdeformation. This elastic deformation of external spline part 123generates a restoring force that presses each tooth 123 b of externalspline part 123 on tooth 113 b of internal spline part 113 adjacent totooth 123 b in the circumferential direction, and thereby generates africtional force that keeps positions of first shaft piece 11 and secondshaft piece 12 in the axial direction.

FIG. 5 is an enlarged longitudinal half-sectional view of secondcollapse connection C2 and its vicinity.

As shown in FIG. 5, according to the above configuration of the splinefitting, internal spline part 113 and external spline part 123 form thesecond collapse connection C2 serving as a second collapse mechanism.When the vehicle collides with something, second collapse connection C2allows first shaft piece 11 and second shaft piece 12 to relatively movein a shortening direction via both spline parts 113 and 123, and therebysuppresses propeller shaft PS from being bent, similar to first collapseconnection C1.

(Production Method for the Propeller Shaft)

The following describes a production method for the propeller shaftaccording to the present embodiment of the present invention, focusingon a process for machining the inner periphery of second tube 112 and aprocess for connecting first shaft piece 11 and second shaft piece 12,with reference to FIGS. 6A to 6D and FIGS. 7A to 7C.

FIGS. 6A, 6B, 6C, and 6D are longitudinal half-sectional views of secondtube 112, illustrating the process for machining the inner periphery ofsecond tube 112, wherein FIG. 6A shows a first larger-radius partcutting operation, FIG. 6B shows a first taper part cutting operation,FIG. 6C shows a second larger-radius part cutting operation, and FIG. 6Dshows a second taper part cutting operation.

The machining of the inner periphery of second tube 112 is started withputting the cutter TL into second tube 112 via its rear end side, andcutting the first larger-radius part 115 by cutter TL, while rotatingthe second tube 112 chucked on a lathe not shown, as shown in FIG. 6A(the first larger-radius part cutting operation). Subsequently, as shownin FIG. 6B, cutter TL is made to travel ahead inward and cut the firsttaper part 114 (the first taper part cutting operation). As describedabove, during formation of first taper part 114, lower-surface-roughnessportion 114 a is formed in the surface of first taper part 114 bymachining at a feed speed slower than that of the first larger-radiuspart cutting operation, or the second larger-radius part cuttingoperation described below, or the second taper part cutting operationdescribed below.

Next, cutter TL is maintained in second tube 112 and made to cut theteeth 113 b of internal spline part 113. Thereafter, cutter TL is putinto second tube 112 via its front end side, i.e. the opposite side inthe axial direction, and is made to cut the second larger-radius part117 as shown in FIG. 6C (the second larger-radius part cuttingoperation). Subsequently, cutter TL is made to travel ahead inward andcut the second taper part 116 as shown in FIG. 6D (the second taper partcutting operation).

After the machining of the inner periphery of second tube 112, secondshaft piece 12 (tubular member 121) is inserted into the innerperipheral side of first shaft piece 11 (second tube 112) via the rearend side of first shaft piece 11 (an insertion process).

FIGS. 7A, 7B, and 7C are views illustrating steps of the process forinserting second shaft piece 12 into first shaft piece 11. In thesedrawings, solid lines and virtual lines respectively represent firstshaft piece 11 (second tube 112) and second shaft piece 12 (tubularmember 121).

Specifically, as shown in FIG. 7A, the rear end of first shaft piece 11(internal spline part 113) and the front end of second shaft piece 12(external spline part 123) are brought face-to-face with each other.Subsequently, as shown in FIG. 7B, from this situation that internalspline part 113 and external spline part 123 have relative angle θxtherebetween, second shaft piece 12 is inserted into first shaft piece11 while twisted in a direction of an arrow in FIG. 7B. This causes anelastic torsion of each tooth 123 b of external spline part 123 in thetooth thickness direction, and brings each tooth 123 b into presscontact with adjacent tooth 113 b of internal spline part 113 forfitting between external spline part 123 and internal spline part 113.Then, as shown in FIG. 7C, internal spline part 113 and external splinepart 123 fit with each other throughout their entire range in the axialdirection, and the connection of first shaft piece 11 and second shaftpiece 12 is thus completed.

Effects of the Embodiment

FIGS. 8A and 8B are enlarged sectional views of a focal portion of firsttaper part 114, wherein FIG. 8A shows a surface of first taper part 114according to a conventional propeller shaft, and FIG. 8B shows a surfaceof first taper part 114 (lower-surface-roughness portion 114 a)according to the present embodiment.

As described above, the conventional propeller shaft is given no regardto the surface roughness of first taper part 114. In other words, theconventional propeller shaft includes first taper part 114 whose surfaceincludes microscopic recesses 114 b and projections 114 c, as shown inFIG. 8A. Accordingly, during insertion (press fitting) of second shaftpiece 12, projections 114 c are liable to bite teeth 123 b of externalspline part 123, wherein projections 114 c are in open edges of grooves113 a of internal spline part 113 open-ended at first taper part 114.This causes a problem that the insertion load of second shaft piece 12disperses depending on presence or absence and extent (magnitude) of thebiting.

Especially in the present embodiment where internal spline part 113 andexternal spline part 123 have relative angle θx therebetween, theprocess of fitting external spline part 123 to internal spline part 113needs application of a twisting force in the circumferential directionalong with application of the insertion load in the axial direction asdescribed above. In reaction to these press forces during insertion ofexternal spline part 123, each tooth 123 b of external spline part 123is applied with an excessive surface pressure from the open edge ofgroove 113 a of internal spline part 113. This causes projections 114 cto bite and scrape a flank of each tooth 123 b of external spline part123, wherein projections 114 c are in the open edges of grooves 113 a ofinternal spline part 113 open-ended at first taper part 114. Thisincreases a frictional force in the direction of insertion of secondshaft piece 12, and accordingly causes the insertion load dispersion.

In contrast, propeller shaft PS according to the present embodimentserves to solve the problem about the conventional propeller shaft byproducing the following beneficial effects.

Propeller shaft PS includes a shaft unit for transmitting a drivingforce from a drive source to a driving wheel, wherein the shaft unitincludes first shaft piece 11 and second shaft piece 12, wherein: firstshaft piece 11 includes second tube 112 at its first end side; secondtube 112 of first shaft piece 11 includes, in its inner periphery,internal spline part 113, first taper part 114, first larger-radius part115, second taper part 116, and second larger-radius part 117; secondshaft piece 12 includes external spline part 123 in the outer peripheryof its first end side, wherein the first end side of second shaft piece12 is structured to be inserted into second tube 112 of first shaftpiece 11 via the first end side of first shaft piece 11, and whereinexternal spline part 123 is structured to fit with internal spline part113; first taper part 114 is formed at the first end of internal splinepart 113 in the direction of rotation axis Z of the shaft unit such thatinner radius Rx of second tube 112 gradually increases in first taperpart 114 from the first end of internal spline part 113 toward the firstend side of second tube 112 in the direction of rotation axis Z and suchthat groove 113 a of internal spline part 113 is open-ended at the firsttaper part 114, wherein inner radius Rx is the distance from rotationaxis Z to the inner periphery of second tube 112; first larger-radiuspart 115 is formed closer to the first end side of second tube 112 inthe direction of rotation axis Z than first taper part 114 such thatfirst larger-radius part 115 has inner radius R1 greater than distanceR3 from rotation axis Z to groove 113 a of internal spline part 113;second taper part 116 is formed at the second end of internal splinepart 113 in the direction of rotation axis Z such that inner radius Rxof second tube 112 gradually increases in second taper part 116 from thesecond end of internal spline part 113 toward the second end side ofsecond tube 112 in the direction of rotation axis Z and such that groove113 a of internal spline part 113 is open-ended at second taper part116; second larger-radius part 117 is formed closer to the second endside of second tube 112 in the direction of rotation axis Z than secondtaper part 116 such that second larger-radius part 117 has the innerradius greater than distance R3 from rotation axis Z to groove 113 a ofinternal spline part 113; and first taper part 114 includeslower-surface-roughness portion 114 a in its surface, whereinlower-surface-roughness portion 114 a is lower in surface roughness thanone of first larger-radius part 115, second larger-radius part 117, andsecond taper part 116.

In this way, as shown especially in FIG. 8B, the present embodiment isconfigured such that first taper part 114 includeslower-surface-roughness portion 114 a to have a reduced surfaceroughness value. This configuration smooths recesses and projections ofthe open edge of each groove 113 a of internal spline part 113open-ended at first taper part 114, and thereby reduces the surfacepressure on each tooth 123 b of external spline part 123 from the openedge. Then, this suppresses the open edge of each groove 113 a ofinternal spline part 113 from biting the tooth 123 b of external splinepart 123, and thereby suppresses the problem that the insertion load ofsecond shaft piece 12 disperses for each product.

Furthermore, according to the present embodiment, internal spline part113 and external spline part 123 are formed to have relative angle θxbetween the direction of the tooth trace of internal spline part 113with respect to rotation axis Z and the direction of the tooth trace ofexternal spline part 123 with respect to rotation axis Z.

Due to this configuration of forming relative angle θx between the toothtrace directions of both spline parts 113 and 123, they fit with eachother along with torsional deformation of their teeth 113 b and 123 b.This improves strength of fitting between both spline parts 113 and 123.

Furthermore, according to the present embodiment, the direction of thetooth trace of internal spline part 113 is parallel to rotation axis Z,and the direction of the tooth trace of external spline part 123 isinclined with respect to rotation axis Z.

This configuration of forming internal spline part 113 parallel torotation axis Z has an advantage to facilitate the machining of internalspline part 113 that is comparatively difficult to machine.

Furthermore, according to the present embodiment, the surface roughnessof lower-surface-roughness portion 114 a has arithmetic mean roughnessvalue Ra of 6 or less.

This configuration of setting the surface roughness (Ra) of first taperpart 114 less than or equal to 6 serves to sufficiently smooth therecesses and projections in the open edge of groove 113 a of internalspline part 113 open-ended at first taper part 114.

Furthermore, according to the present embodiment, external spline part123 includes tooth 123 b structured to be inserted into groove 113 a ofinternal spline part 113 within its region of elastic deformation.

This configuration of limiting the deformation of tooth 123 b ofexternal spline part 123 within the region of elastic deformation servesto further suppress the insertion load dispersion for each product.

Furthermore, according to the present embodiment, first taper part 114is formed such that minor angle θ1 between first taper part 114 androtation axis Z is less than minor angle θ2 between second taper part116 and rotation axis Z.

The smaller inclination angle (θ1) of first taper part 114 facilitatesthe insertion of second shaft piece 12. On the other hand, the largerinclination angle (θ2) of second taper part 116 serves to shorten secondtaper part 116 in the axial direction.

Furthermore, according to the present embodiment: second shaft piece 12includes third taper part 125 in its outer periphery; third taper part125 is formed at the first end of external spline part 123 such that theouter radius of second shaft piece 12 gradually decreases in third taperpart 125 from the first end of external spline part 123 toward the firstend side of second shaft piece 12 and such that groove 123 a of externalspline part 123 is open-ended at third taper part 125, wherein the outerradius is the distance from rotation axis Z to the outer periphery ofsecond shaft piece 12; and lower-surface-roughness portion 114 a is setlower in surface roughness than third taper part 125.

This configuration of setting third taper part 125 of second shaft piece12 greater in surface roughness than first taper part 114 allows thirdtaper part 125 to be easily machined.

Furthermore, propeller shaft PS includes the shaft unit for transmittinga driving force from the drive source to the driving wheel, wherein theshaft unit includes first shaft piece 11 and second shaft piece 12,wherein: first shaft piece 11 includes second tube 112 at its first endside; second tube 112 of first shaft piece 11 includes internal splinepart 113 and first taper part 114 in its inner periphery; second shaftpiece 12 includes external spline part 123 in the outer periphery of itsfirst end side, wherein the first end side of second shaft piece 12 isstructured to be inserted into second tube 112 of first shaft piece 11via the first end side of first shaft piece 11, and wherein externalspline part 123 is structured to fit with internal spline part 113;first taper part 114 is formed at the first end of internal spline part113 in the direction of rotation axis Z of the shaft unit such thatinner radius Rx of second tube 112 gradually increases in first taperpart 114 from the first end of internal spline part 113 toward the firstend side of second tube 112 in the direction of rotation axis Z and suchthat groove 113 a of internal spline part 113 is open-ended at firsttaper part 114, wherein inner radius Rx is the distance from rotationaxis Z to the inner periphery of second tube 112; and first taper part114 includes lower-surface-roughness portion 114 a in its surface,wherein surface roughness of lower-surface-roughness portion 114 a hasarithmetic mean roughness value Ra of 6 or less.

In this way, as shown especially in FIG. 8B, the present embodiment isconfigured such that the surface of first taper part 114 is processed tohave a surface roughness value (Ra) of 6 or less. This configurationsmooths the recesses and projections of the open edge of groove 113 a ofinternal spline part 113 open-ended at first taper part 114, and therebyreduces the surface pressure on each tooth 123 b of external spline part123 from the open edge. Then, this suppresses the open edge of eachgroove 113 a of internal spline part 113 from biting the tooth 123 b ofexternal spline part 123, and thereby suppresses the problem that theinsertion load of second shaft piece 12 disperses for each product.

Furthermore, according to the present embodiment, the production methodfor propeller shaft PS including the shaft unit for transmitting adriving force from the drive source to the driving wheel, wherein theshaft unit includes first shaft piece 11 and second shaft piece 12,wherein: first shaft piece 11 includes second tube 112 at its first endside; second tube 112 of first shaft piece 11 includes, in its innerperiphery, internal spline part 113, first taper part 114, firstlarger-radius part 115, second taper part 116, and a secondlarger-radius part 117; first taper part 114 is formed at the first endof internal spline part 113 in the direction of rotation axis Z of theshaft unit such that inner radius Rx of second tube 112 graduallyincreases in first taper part 114 from the first end of internal splinepart 113 toward the first end side of second tube 112 in the directionof rotation axis Z and such that groove 113 a of internal spline part113 is open-ended at first taper part 114, wherein inner radius Rx isthe distance from rotation axis Z to the inner periphery of second tube112; first larger-radius part 115 is formed closer to the first end sideof second tube 112 in the direction of rotation axis Z than first taperpart 114 such that first larger-radius part 115 has inner radius R1greater than distance R3 from rotation axis Z to groove 113 a ofinternal spline part 113; second taper part 116 is formed at the secondend of internal spline part 113 in the direction of rotation axis Z suchthat inner radius Rx of second tube 112 gradually increases in secondtaper part 116 from the second end of internal spline part 113 towardthe second end side of second tube 112 in the direction of rotation axisZ and such that groove 113 a of internal spline part 113 is open-endedat second taper part 116; second larger-radius part 117 is formed closerto the second end side of second tube 112 in the direction of rotationaxis Z than second taper part 116 such that second larger-radius part117 has the inner radius greater than distance R3 from rotation axis Zto groove 113 a of internal spline part 113; and second shaft piece 12includes external spline part 123 in the outer periphery of its firstend side, wherein the first end side of second shaft piece 12 isstructured to be inserted into second tube 112 of first shaft piece 11via the first end side of first shaft piece 11, and wherein externalspline part 123 is structured to fit with internal spline part 113; theproduction method includes: the first taper part cutting operation inwhich the surface of first taper part 114 is cut by machining to belower in surface roughness than one of second taper part 116, firstlarger-radius part 115, and second larger-radius part 117; the secondtaper part cutting operation in which the surface of second taper part116 is cut by machining; the first larger-radius part cutting operationin which the surface of first larger-radius part 115 is cut bymachining; the second larger-radius part cutting operation in which thesurface of second larger-radius part 117 is cut by machining; and theinsertion operation in which second shaft piece 12 is inserted intosecond tube 112 of first shaft piece 11 via the first end side of firstshaft piece 11.

In this way, as shown especially in FIG. 8B, the present embodiment isconfigured such that first taper part 114 is processed to havelower-surface-roughness portion 114 a, and thereby have a low surfaceroughness value. This configuration smooths the recesses and projectionsof the open edge of groove 113 a of internal spline part 113 open-endedat first taper part 114, and thereby reduces the surface pressure oneach tooth 123 b of external spline part 123 from the open edge. Then,this suppresses the open edge of each groove 113 a of internal splinepart 113 from biting the tooth 123 b of external spline part 123, andthereby suppresses the problem that the insertion load of second shaftpiece 12 disperses for each product.

Furthermore, according to the present embodiment, the first taper partcutting operation is implemented by machining at a slower feed speedthan one of the second taper part cutting operation, the firstlarger-radius part cutting operation, and the second larger-radius partcutting operation.

This configuration of setting slower the feed speed of machining offirst taper part 114 during the first taper part cutting operation,serves to reduce the surface roughness of first taper part 114. In otherwords, this configuration eliminates necessity of a process forfinishing first taper part 114 after the machining, and minimizes anadverse effect of the formation of lower-surface-roughness portion 114 aon its productivity, and serves to suppress its production cost frombeing increased.

Furthermore, according to the present embodiment, the first taper partcutting operation and the first larger-radius part cutting operation areimplemented in succession by using the same tool (cutter TL).

This configuration that the machining of the first taper part cuttingoperation and the machining of the first larger-radius part cuttingoperation are implemented in succession with the same cutter TL whilevarying the feed speed, serves to form the lower-surface-roughnessportion 114 a with a reduced surface roughness.

The present invention is not limited to the configurations nor modesexemplified in the above embodiment, but may be freely modifiedaccording to requirements in specifications, cost, etc. of anapplication target, provided that the application target is capable ofproducing the effects of the present invention described above.

The propeller shaft according to the above embodiment may be exemplifiedas follows.

The propeller shaft includes, according to its one aspect, a shaft unitfor transmitting a driving force from a drive source to a driving wheel,wherein the shaft unit includes a first shaft piece and a second shaftpiece, wherein: the first shaft piece includes a tubular section at itsfirst end side; the tubular section of the first shaft piece includes,in its inner periphery, an internal spline part, a first taper part, afirst larger-radius part, a second taper part, and a secondlarger-radius part; the second shaft piece includes an external splinepart in an outer periphery of its first end side, wherein the first endside of the second shaft piece is structured to be inserted into thetubular section of the first shaft piece via the first end side of thefirst shaft piece, and wherein the external spline part is structured tofit with the internal spline part; the first taper part is formed at afirst end of the internal spline part in a direction of a rotation axisof the shaft unit such that an inner radius of the tubular sectiongradually increases in the first taper part from the first end of theinternal spline part toward a first end side of the tubular section inthe direction of the rotation axis and such that a groove of theinternal spline part is open-ended at the first taper part, wherein theinner radius is a distance from the rotation axis to the inner peripheryof the tubular section; the first larger-radius part is formed closer tothe first end side of the tubular section in the direction of therotation axis than the first taper part such that the firstlarger-radius part has an inner radius greater than a distance from therotation axis to the groove of the internal spline part; the secondtaper part is formed at a second end of the internal spline part in thedirection of the rotation axis such that the inner radius of the tubularsection gradually increases in the second taper part from the second endof the internal spline part toward a second end side of the tubularsection in the direction of the rotation axis and such that the grooveof the internal spline part is open-ended at the second taper part; thesecond larger-radius part is formed closer to the second end side of thetubular section in the direction of the rotation axis than the secondtaper part such that the second larger-radius part has an inner radiusgreater than the distance from the rotation axis to the groove of theinternal spline part; and the first taper part includes alower-surface-roughness portion in its surface, wherein thelower-surface-roughness portion is lower in surface roughness than oneof the first larger-radius part, the second larger-radius part, and thesecond taper part.

According to an exemplary aspect of this propeller shaft, the internalspline part and the external spline part are formed to have a relativeangle between a direction of a tooth trace of the internal spline partwith respect to the rotation axis and a direction of a tooth trace ofthe external spline part with respect to the rotation axis.

According to another exemplary aspect of the propeller shaft along withany one of the above aspects, the direction of the tooth trace of theinternal spline part is parallel to the rotation axis, and wherein thedirection of the tooth trace of the external spline part is inclinedwith respect to the rotation axis.

According to a further exemplary aspect of the propeller shaft alongwith any one of the above aspects, the surface roughness of thelower-surface-roughness portion has an arithmetic mean roughness valueRa of 6 or less.

According to a further exemplary aspect of the propeller shaft alongwith any one of the above aspects, the external spline part includes atooth structured to be inserted into the groove of the internal splinepart within its region of elastic deformation.

According to a further exemplary aspect of the propeller shaft alongwith any one of the above aspects, the first taper part is formed suchthat a minor angle between the first taper part and the rotation axis isless than a minor angle between the second taper part and the rotationaxis.

According to a further exemplary aspect of the propeller shaft alongwith any one of the above aspects: the second shaft piece includes athird taper part in its outer periphery; the third taper part is formedat a first end of the external spline part such that an outer radius ofthe second shaft piece gradually decreases in the third taper part fromthe first end of the external spline part toward the first end side ofthe second shaft piece and such that a groove of the external splinepart is open-ended at the third taper part, wherein the outer radius isa distance from the rotation axis to the outer periphery of the secondshaft piece; and the lower-surface-roughness portion is set lower insurface roughness than the third taper part.

From another point of view about the above embodiment, the propellershaft includes, according to its one aspect, a shaft unit fortransmitting a driving force from a drive source to a driving wheel,wherein the shaft unit includes a first shaft piece and a second shaftpiece, wherein: the first shaft piece includes a tubular section at itsfirst end side; the tubular section of the first shaft piece includes aninternal spline part and a taper part in its inner periphery; the secondshaft piece includes an external spline part in an outer periphery ofits first end side, wherein the first end side of the second shaft pieceis structured to be inserted into the tubular section of the first shaftpiece via the first end side of the first shaft piece, and wherein theexternal spline part is structured to fit with the internal spline part;the taper part is formed at a first end of the internal spline part in adirection of a rotation axis of the shaft unit such that an inner radiusof the tubular section gradually increases in the taper part from thefirst end of the internal spline part toward a first end side of thetubular section in the direction of the rotation axis and such that agroove of the internal spline part is open-ended at the taper part,wherein the inner radius is a distance from the rotation axis to theinner periphery of the tubular section; and the taper part includes alower-surface-roughness portion in its surface, wherein surfaceroughness of the lower-surface-roughness portion has an arithmetic meanroughness value Ra of 6 or less.

Furthermore, the production method for the propeller shaft according tothe above embodiment may be exemplified as follows.

According to one aspect of the production method, the propeller shaftincludes a shaft unit for transmitting a driving force from a drivesource to a driving wheel, wherein the shaft unit includes a first shaftpiece and a second shaft piece, wherein: the first shaft piece includesa tubular section at its first end side; the tubular section of thefirst shaft piece includes, in its inner periphery, an internal splinepart, a first taper part, a first larger-radius part, a second taperpart, and a second larger-radius part; the first taper part is formed ata first end of the internal spline part in a direction of a rotationaxis of the shaft unit such that an inner radius of the tubular sectiongradually increases in the first taper part from the first end of theinternal spline part toward a first end side of the tubular section inthe direction of the rotation axis and such that a groove of theinternal spline part is open-ended at the first taper part, wherein theinner radius is a distance from the rotation axis to the inner peripheryof the tubular section; the first larger-radius part is formed closer tothe first end side of the tubular section in the direction of therotation axis than the first taper part such that the firstlarger-radius part has an inner radius greater than a distance from therotation axis to the groove of the internal spline part; the secondtaper part is formed at a second end of the internal spline part in thedirection of the rotation axis such that the inner radius of the tubularsection gradually increases in the second taper part from the second endof the internal spline part toward a second end side of the tubularsection in the direction of the rotation axis and such that the grooveof the internal spline part is open-ended at the second taper part; thesecond larger-radius part is formed closer to the second end side of thetubular section in the direction of the rotation axis than the secondtaper part such that the second larger-radius part has an inner radiusgreater than the distance from the rotation axis to the groove of theinternal spline part; and the second shaft piece includes an externalspline part in an outer periphery of its first end side, wherein thefirst end side of the second shaft piece is structured to be insertedinto the tubular section of the first shaft piece via the first end sideof the first shaft piece, and wherein the external spline part isstructured to fit with the internal spline part; the production methodincludes: a first taper part cutting operation in which a surface of thefirst taper part is cut by machining to be lower in surface roughnessthan one of the second taper part, the first larger-radius part, and thesecond larger-radius part; a second taper part cutting operation inwhich a surface of the second taper part is cut by machining; a firstlarger-radius part cutting operation in which a surface of the firstlarger-radius part is cut by machining; a second larger-radius partcutting operation in which a surface of the second larger-radius part iscut by machining; and an insertion operation in which the second shaftpiece is inserted into the tubular section of the first shaft piece viathe first end side of the first shaft piece.

According to an exemplary aspect of this production method, the firsttaper part cutting operation is implemented by machining at a slowerfeed speed than one of the second taper part cutting operation, thefirst larger-radius part cutting operation, and the second larger-radiuspart cutting operation.

According to another exemplary aspect of the production method alongwith any one of the above aspects, the first taper part cuttingoperation and the first larger-radius part cutting operation areimplemented in succession by using a same tool.

The invention claimed is:
 1. A propeller shaft comprising: a shaft unitfor transmitting a driving force from a drive source to a driving wheel,wherein the shaft unit includes a first shaft piece and a second shaftpiece, wherein: the first shaft piece includes a tubular section at itsfirst end side; the tubular section of the first shaft piece includes,in its inner periphery, an internal spline part, a first taper part, afirst larger-radius part, a second taper part, and a secondlarger-radius part; the second shaft piece includes an external splinepart in an outer periphery of its first end side, wherein the first endside of the second shaft piece is structured to be inserted into thetubular section of the first shaft piece via the first end side of thefirst shaft piece, and wherein the external spline part is structured tofit with the internal spline part; the first taper part is formed at afirst end of the internal spline part in a direction of a rotation axisof the shaft unit such that an inner radius of the tubular sectiongradually increases in the first taper part from the first end of theinternal spline part toward a first end side of the tubular section inthe direction of the rotation axis and such that a groove of theinternal spline part is open-ended at the first taper part, wherein theinner radius is a distance from the rotation axis to the inner peripheryof the tubular section; the first larger-radius part is formed closer tothe first end side of the tubular section in the direction of therotation axis than the first taper part such that the firstlarger-radius part has an inner radius greater than a distance from therotation axis to the groove of the internal spline part; the secondtaper part is formed at a second end of the internal spline part in thedirection of the rotation axis such that the inner radius of the tubularsection gradually increases in the second taper part from the second endof the internal spline part toward a second end side of the tubularsection in the direction of the rotation axis and such that the grooveof the internal spline part is open-ended at the second taper part; thesecond larger-radius part is formed closer to the second end side of thetubular section in the direction of the rotation axis than the secondtaper part such that the second larger-radius part has an inner radiusgreater than the distance from the rotation axis to the groove of theinternal spline part; and the first taper part includes alower-surface-roughness portion in its surface, wherein thelower-surface-roughness portion is lower in surface roughness than oneof the first larger-radius part, the second larger-radius part, or thesecond taper part.
 2. The propeller shaft according to claim 1, whereinthe internal spline part and the external spline part are formed to havea relative angle between a direction of a tooth trace of the internalspline part with respect to the rotation axis and a direction of a toothtrace of the external spline part with respect to the rotation axis. 3.The propeller shaft according to claim 2, wherein the direction of thetooth trace of the internal spline part is parallel to the rotationaxis, and wherein the direction of the tooth trace of the externalspline part is inclined with respect to the rotation axis.
 4. Thepropeller shaft according to claim 3, wherein: the second shaft pieceincludes a third taper part in its outer periphery; the third taper partis formed at a first end of the external spline part such that an outerradius of the second shaft piece gradually decreases in the third taperpart from the first end of the external spline part toward the first endside of the second shaft piece and such that a groove of the externalspline part is open-ended at the third taper part, wherein the outerradius is a distance from the rotation axis to the outer periphery ofthe second shaft piece; and the lower-surface-roughness portion is setlower in surface roughness than the third taper part.
 5. The propellershaft according to claim 1, wherein the surface roughness of thelower-surface-roughness portion has an arithmetic mean roughness valueRa of 6 μm or less.
 6. The propeller shaft according to claim 1, whereinthe external spline part includes a tooth structured to be inserted intothe groove of the internal spline part within its region of elasticdeformation.
 7. The propeller shaft according to claim 1, wherein thefirst taper part is formed such that a minor angle between the firsttaper part and the rotation axis is less than a minor angle between thesecond taper part and the rotation axis.
 8. A propeller shaftcomprising: a shaft unit for transmitting a driving force from a drivesource to a driving wheel, wherein the shaft unit includes a first shaftpiece and a second shaft piece, wherein: the first shaft piece includesa tubular section at its first end side; the tubular section of thefirst shaft piece includes an internal spline part and a taper part inits inner periphery; the second shaft piece includes an external splinepart in an outer periphery of its first end side, wherein the first endside of the second shaft piece is structured to be inserted into thetubular section of the first shaft piece via the first end side of thefirst shaft piece, and wherein the external spline part is structured tofit with the internal spline part; the taper part is formed at a firstend of the internal spline part in a direction of a rotation axis of theshaft unit such that an inner radius of the tubular section graduallyincreases in the taper part from the first end of the internal splinepart toward a first end side of the tubular section in the direction ofthe rotation axis and such that a groove of the internal spline part isopen-ended at the taper part, wherein the inner radius is a distancefrom the rotation axis to the inner periphery of the tubular section;and the taper part includes a lower-surface-roughness portion in itssurface, wherein surface roughness of the lower-surface-roughnessportion has an arithmetic mean roughness value Ra of 6 μm or less. 9.The propeller shaft according to claim 8, wherein the external splinepart includes a tooth structured to be inserted into the groove of theinternal spline part within its region of elastic deformation.
 10. Aproduction method for a propeller shaft, the propeller shaft comprising:a shaft unit for transmitting a driving force from a drive source to adriving wheel, wherein the shaft unit includes a first shaft piece and asecond shaft piece, wherein: the first shaft piece includes a tubularsection at its first end side; the tubular section of the first shaftpiece includes, in its inner periphery, an internal spline part, a firsttaper part, a first larger-radius part, a second taper part, and asecond larger-radius part; the first taper part is formed at a first endof the internal spline part in a direction of a rotation axis of theshaft unit such that an inner radius of the tubular section graduallyincreases in the first taper part from the first end of the internalspline part toward a first end side of the tubular section in thedirection of the rotation axis and such that a groove of the internalspline part is open-ended at the first taper part, wherein the innerradius is a distance from the rotation axis to the inner periphery ofthe tubular section; the first larger-radius part is formed closer tothe first end side of the tubular section in the direction of therotation axis than the first taper part such that the firstlarger-radius part has an inner radius greater than a distance from therotation axis to the groove of the internal spline part; the secondtaper part is formed at a second end of the internal spline part in thedirection of the rotation axis such that the inner radius of the tubularsection gradually increases in the second taper part from the second endof the internal spline part toward a second end side of the tubularsection in the direction of the rotation axis and such that the grooveof the internal spline part is open-ended at the second taper part; thesecond larger-radius part is formed closer to the second end side of thetubular section in the direction of the rotation axis than the secondtaper part such that the second larger-radius part has an inner radiusgreater than the distance from the rotation axis to the groove of theinternal spline part; and the second shaft piece includes an externalspline part in an outer periphery of its first end side, wherein thefirst end side of the second shaft piece is structured to be insertedinto the tubular section of the first shaft piece via the first end sideof the first shaft piece, and wherein the external spline part isstructured to fit with the internal spline part; the production methodcomprising: a first taper part cutting operation in which a surface ofthe first taper part is cut by machining to be lower in surfaceroughness than one of the second taper part, the first larger-radiuspart, or the second larger-radius part; a second taper part cuttingoperation in which a surface of the second taper part is cut bymachining; a first larger-radius part cutting operation in which asurface of the first larger-radius part is cut by machining; a secondlarger-radius part cutting operation in which a surface of the secondlarger-radius part is cut by machining; and an insertion operation inwhich the second shaft piece is inserted into the tubular section of thefirst shaft piece via the first end side of the first shaft piece. 11.The production method according to claim 10, wherein the first taperpart cutting operation is implemented by machining at a slower feedspeed than one of the second taper part cutting operation, the firstlarger-radius part cutting operation, or the second larger-radius partcutting operation.
 12. The production method according to claim 11,wherein the first taper part cutting operation and the firstlarger-radius part cutting operation are implemented in succession byusing a same tool.